The invention relates to an apparatus for calibration of temperature sensors, comprising a cavity for receiving a sensor to be calibrated, a heat-transferring medium for surrounding the sensor in the cavity, and a means for heating/cooling of the medium to a desired temperature.
As components and equipment age, and also are subjected to temperature changes and mechanical strains, the critical capacity will decrease. This is called drift. When this happens, one can no longer rely on previous capacity tests, and this may result in poorer quality in development and production. Since one can not prevent drift, one must instead detect and eliminate the effect thereof by means of calibration.
As known, calibration is to compare the capacity of an equipment with a known standard. The standards are maintained by national and international accredited calibration laboratories.
Today, there are imposed aggravated demands on the industry with respect to measuring accuracy and quality assurance (e.g., ISO 9000), and there is then required calibrating equipment which satisfies the requirement of today and complies with the new standards within temperature calibration.
The prior art traditionally includes two principal types of calibrators, namely liquid bath calibrators and dry block calibrators.
In the liquid bath calibrator, the sensor to be calibrated is lowered into a liquid during forced agitation at a given reference temperature. A calibrator of this type has advantages and drawbacks which may be summarized as follows:
Advantages:
A good and defined heat transfer/temperature difference between liquid and reference thermometer and between liquid and sensor/thermometer to be calibrated
A liquid during agitation has a smooth temperature distribution and therefore is both control-technically advantageous and able to calibrate sensors with large differences in length
Drawbacks:
A liquid possibly will have to be changed for different temperature ranges
A liquid may result in spill and damages, especially at extreme temperatures
The temperature sensor has to be dried, and possibly cleaned after use
A liquid is a drawback in case of transport, and in portable solutions is disadvantageous with the respect to handling.
In the dry block calibrator, the sensor is placed in a bore which is then air-filled. The ambient temperature of the bore is regulated to a temperature which is as close as possible to the desired reference temperature. Drawbacks and advantages of this type of calibrator may be summarized as follows:
Advantages:
Simple and cleanly handling
Well suited for portable devices and therefore for in situ calibration
Drawbacks:
Heat transfer air/sensor with natural convection is slow and difficult to define physically, and the percentage of radiation is obscure so that the temperature difference between reference and sensor is indistinctly defined in time
Sensitive to varying thermal capacity in the temperature sensors to be calibrated
Exacting optimization between geometry and mass in the block in order to achieve a good temperature distribution, a uniform temperature in the bore and small masses in order to change the temperature picture quickly
Requires a good cleaning of block and bore for maintaining the thermal properties influencing the accuracy
Control-technically exacting
Difficult to achieve the same temperature in the entire depth of the bore
Greater measuring uncertainty than for the liquid bath calibrator An apparatus of the introductorily stated type is known e.g. from the international patent application No. PCT/NO98/00218. This calibrator possesses most of the advantages of both of the above-mentioned principal types, but still has the drawbacks of a large mass and weight in the calibrator block, and with the liquid-filled liner it has the same cleaning problems as the liquid bath calibrator.
It is a principal object of the invention to provide a calibrating apparatus which combines the high accuracy of the liquid bath calibrator with the simplicity and environmental advantages (cleanliness) in use of the dry block calibrator.
A more particular object of the invention is to provide a calibrating apparatus with which there is obtained a physically well definable, desired reference temperature and a quick regulation thereof
Further objects of the invention is to provide such an apparatus allowing
achievement of the smallest possible weight and smallest possible dimensions for the entire instrument
achievement of the lowest possible energy demand
minimized risk for personal injuries because of contact with very hot or cold liquids or components
coverage of a large temperature range with the smallest possible change of construction and instrument variants.
The above-mentioned objects are achieved with a calibrating apparatus of the introductorily stated type which, according to the invention, is characterized in that the cavity receives at least one sensor-surrounding thin-walled calibrator body which is made of a pliable material having a high thermal conductivity and which defines a volume which is filled of the heat-transferring medium, the calibrator body having an inner wall defining at least a part of an inner opening for receiving at least one sensor, and being arranged to be subjected to a pressure influence to bring the opening-defining wall of the calibrator body into tight-fitting abutment against the sensor or sensors.
The heat-transferring medium in the apparatus may be a liquid or a gas. The material of the calibrator body advantageously may be elastic.
An advantageous embodiment of the apparatus is characterized in that the side of the calibrator body wall facing the heat-transferring medium, has a surface-increasing structure ensuring maximum heat transfer, whereas the side of the wall facing a sensor body has a structure ensuring an optimum contact surface and heat transfer.
The pressure of the gas or liquid filling is adjusted such that the filling may get an underpressure or no pressure difference relative to the surroundings, so that a sufficient opening is obtained when introducing the temperature sensor, whereas an overpressure is applied during the calibration to secure that the calibrator body obtains a maximum contact with the sensor. The pressure of the liquid or gas filling may also be adjusted such that it may facilitate the introduction of the sensor with complete or partial contact of the calibrator bodies.
The overpressure in the calibrator body or calibrator bodies may be applied either in that the liquid or gas filling is supplied with an overpressure directly, in that adjacent or surrounding bodies exert an external pressure on the calibrator body, or in that an external pressure is applied by means of other mechanical devices.
The liquid or gas filling is subjected to a forced through-flow and passes a region outside the calibrator bodies where heat energy is either supplied or carried off in order to obtain the desired reference temperature.
One or more temperature sensors is/are placed at appropriate places in the liquid or gas flow as transmitters to a control circuit controlling supply or removal of heat energy, possibly together with control of the flow rate, in order to obtain the desired reference temperature in the surrounding calibrator body or bodies.
In order to avoid an undesired supply or removal of heat energy, all liquid or gas-filled parts are provided with a suitable heat insulation.
The apparatus according to the invention allows a compact construction with smaller dimensions and a lower weight than previously known solutions, and it will probably be able to be developed to such a low energy consumption that it may be equipped with a portable current source of its own. The invention thus will be particularly well suited for portable temperature calibrators.